1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to an alkali metal electrochemical cell having a cathode current collector provided with a conductive coating to increase the electrode""s conductivity. A preferred cathode includes a solid active material and a most preferred cathode active material is a metal oxide, such as silver vanadium oxide (SVO).
2. Prior Art
It is known to coat a cathode current collector screen before contacting the active material thereto. Carbonaceous materials are suitable for this purpose. For example, U.S. Pat. No. 6,451,483 to Probst et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes a prior art Li/CFx cell including a titanium cathode current collector screen coated with a thin layer of graphite/carbon paint. In addition to increasing the electrical conductivity between the CFx and the current collector, the graphite/carbon paint serves to prevent direct contact at their interface. A major problem with CFx active materials supported on titanium current collectors is that the fluorine reacts with titanium to form titanium fluoride. This by-product is corrosive and degrades cell discharge performance through current collector pitting.
For example, U.S. Pat. No. 6,261,722 to Dasgupta et al. disclose a lithium battery comprising a cathode current collector provided with a mixture of carbon fibres and fine carbon particles. The problem is that these coating materials are added to a fluorinated polymer and possibly a low boiling point solvent such as toluene, acetone, or n-methyl pyrrolidinine (NMP). The coating is brushed or dipped onto the current collector and then evaporated off before being contacted by the cathode active material. Dasgupta et al. list a host of suitable current collector materials including titanium. However, according o the present invention, the presence of fluorine ions in contact with the titanium current collector detracts from cell performance, especially as the cell approaches end-of-life (EOL).
Accordingly, the present invention is directed to coating a titanium substrate with a carbonaceous material. When the substrate is used as a current collector, this increases the electrical conductivity at the active material/substrate interface. Preferably, the carbonaceous material in an alcohol-based suspension is contacted to the substrate followed by a heating step to affect the bonding process. The thusly-processed substrates are useful as a cathode current collector in a lithium/solid cathode active material cell. Preferred cell chemistry is of a Li/SVO couple.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.